Electro-mechanical switches are utilized in a variety of industrial, consumer and commercial applications. Certain types of electrical switching applications require a mechanical switch that can operate properly with a slowly applied, low-actuation force. Such a switch must also be extremely reliable and generate an accurate, repeatable response, while possessing a small actuation differential. These requirements arise perhaps most commonly in applications involving electro-mechanical thermostats, which are utilized for controlling heating and cooling in homes and buildings where coils of standard bi-metal strips form the switch actuation elements. For many years this thermostatic switching function has been performed by mercury bulb switch elements.
Due to the environmental concerns associated with the use of mercury, it is anticipated that electromechanical switches will eventually replace mercury-based switches. Legislation currently being drafted and passed in a variety of countries, including the United States, is aimed at banning the use of mercury in most consumer-based applications. Thus, non-mercury based switches must be developed to replace such mercury-type switching mechanisms.
Some attempts have been made at replacing mercury-switching devices, but such attempts have not been very successful. For example, so-called “snap action” switches have been designed to address the environmental concerns that mercury bulb switch elements raise. As utilized herein, the term “snap action switch” generally refers to a low actuation force switch, which utilizes an internal mechanism to rapidly shift or snap the movable contact from one position to another thus making or breaking electrical conduction between the movable contact and a fixed contact in response to moving an operating element of the switch, such as a plunger, a lever, a spring, or the like from a first to a second position. Typically, these switches require only a few millimeters of movement by the operating element to change the conduction state of the switch.
Such switches can safely and reliably operate at a current level of several amperes using the standard 24 VAC power that thermostats control. However, when actuated by a slowly-applied, low-actuation force such as is provided by a thermostat's coiled bi-metal strip, snap action switches may occasionally hang in a state between the two conducting states, or may switch so slowly between the two conducting states that unacceptable arcing and/or heat-rise can occur when entering the non-conducting state. Either condition gives rise to unacceptable reliability and predictability of operation.
Furthermore, such switches frequently possess unacceptably large differentials, which means that the position of the operating element at which actuation of the switch to one state occurs differs substantially from the position of the actuation element at which actuation of the switch to the other state occurs. If the differential is too large, then the temperature range that the controlled space experiences is also too large. Accordingly, the use of snap action switches in thermostat-type applications has not been particularly successful.
Electronic thermostats are generally known in the art. An example of an electro-mechanical thermostat that has been utilized in commercial, consumer and industrial applications is the T87 thermostat produced by Honeywell International, Inc. (“Honeywell”). An example of the T87 thermostat is disclosed in the publication “Thermostats T87F,” Form Number 60-2222-2, S. M. Rev. April 1986, which is incorporated herein by reference. Another example of the T87F thermostat is disclosed in the publication “T87F Universal Thermostat,” Form Number 60-0830-3, S. M. Rev. August 1993, which is also incorporated herein by reference. The T87F thermostat, in particular, provides temperature control for residential heating, cooling or heating-cooling systems.
An example of a switch assembly which in various forms can be utilized in thermostat applications is disclosed in U.S. Pat. No. 6,720,852, “Methods and Apparatus for Actuating and Deactuating a Switching Device Using Magnets” which issued on Apr. 13, 2004 to Farrey et al, and which is assigned to Honeywell International Inc. U.S. Pat. No. 6,720,852 is incorporated herein by reference.
One of the problems encountered in the efficient utilization of many thermostats in use today is the problem of actuating an electro-mechanical switch with a slow-moving actuator, such as a bi-metal coil, without sacrificing the switch's electrical life. For example, mechanical thermostats, such as the T87 line of thermostats manufactured by Honeywell, utilize a bi-metal coil as the temperature-sensing device. In the operation of the thermostat, the bi-metal coil moves a small amount at a slow rate.
Actuating a switch directly off the bi-metal coil results in an inordinate amount of time spent, during the switching cycle, at or near snap-over. Electro-mechanical switches have low contact forces near snap-over and zero contact forces at snap-over. When the switch contact forces are low or zero, the amount of electrical resistance at the contact interface increases. As the electrical resistance to current passing through the switch increases, the heat also increases. The electrical life of an electro-mechanical switch is reduced with time as the current is carried at or near the snap-over points.
Unacceptable electrical switching performance can occur in switching applications where the actuating force is resilient in nature and varies for indefinite periods of time slightly below the switch operate force or slightly above the switch release force. Switches are frequently designed and adapted for use with devices that monitor and/or control changes in attributes such as position, pressure, temperature, acceleration, and the like.
In many of these applications, the sensing element that interfaces between the switch mechanism and the system attribute being sensed is resilient. For example, temperature changes can be translated into movement via a coiled bimetal spring. The coiled bimetal responds to temperature changes and acts as a force sensitive (resilient) actuator to drive the switch mechanism. The bimetal is resilient because its position is dependent on force, or vice versa; whereas, a non-resilient (rigid) actuator does not change position as the force against it varies.
Other examples of switch actuators with resilient force-deflection spring rates include bellows, bourdon tubes, diaphragms, floats, clampers, and magnets. Fixed masses, gravitational and non-gravitational accelerations can also be used to create switch actuators that have a resilient nature. If a mass is attached to the external lever of a switch and the switch, external lever, and attached mass assembly is rotated about an axis that is not parallel to the force due to gravity or coincident with the mass's center of gravity, the resulting moment that actuates the switch is a function of the angle of rotation of the switch, external lever, and mass assembly.
In general, if a fixed mass is placed against the switch mechanism and accelerated, a force can be exerted, which is approximately equal to the product of the mass and the acceleration in a direction opposite to the acceleration. Because the mass is fixed, the actuating force against the switch mechanism is a function of acceleration. In all of these applications, the resilient interface responds to a change in stimulus (pressure, temperature, acceleration, etc.) and moves to drive a switch mechanism through some travel range to energize an electrical circuit.
For reliable and predictable electrical switching performance, it is desirable to maintain maximum contact force until the point of actuation or de-actuation. In non-snap switches and the vast majority of precision, snap-action switches, contact forces are at a maximum at the plunger free position (plunger fully extended) and the full over-travel position (plunger fully depressed). Contact force diminishes to zero as the switch mechanism approaches the operating point, the plunger position at which the switch changes electrical state from the normally-closed (NC) circuit to the normally-open circuit (NO).
Likewise, contact force decreases to zero as the switch mechanism approaches its release point, the plunger position at which the switch changes state from the NO circuit back to the NC circuit. As the contact force varies at or near zero, the switch is susceptible to intermittent non-contact, welding of contacts, and excessive heat generation and contact erosion. In addition, once actuation or de-actuation commences, it is desirable that the switch mechanism moves from free position to full over-travel position, or vice versa, in one continuous motion. The uninterrupted motion of the switch mechanism from free position to full over travel results in a minimum amount of time spent near zero contact force and in a maximum relative movement between the moveable contact and the stationary contacts between switching events.
Precision snap-switches, which typically possess a positive rate force-deflection behavior, are commonly utilized in applications where the actuating force is resilient in nature. The plunger force of a positive rate snap switch, for example, can increase as the plunger is depressed from free position to the operating position. Because of the increase in force required to continue plunger movement when a positive rate switch is actuated with a resilient actuator, a balance of forces can occur. The balance of force between the resilient actuator and positive rate switch mechanism becomes unbalanced when the resilient actuator responds to a change in the attribute being sensed.
Changes in the sensed attribute result in an increase or decrease of the force generated by the resilient actuator. Often times the sensed attribute changes very slowly over time, as in the case of a thermostatic bimetal. This results in the switch plunger moving in small increments over a long period of time; which can cause erratic non-contact (dead break), arcing, or welding of the electrical contacts. If the resilient actuator and switch mechanism forces remain balanced, the switch plunger will not move. The balanced condition is detrimental to the electrical switching performance if the switch mechanism is in a position where the contact forces are very low. With some positive rate switches, the resilient actuator force and switch mechanism force can balance during the plunger movement from full over travel to release point; resulting in poor electrical switching performance.